1. Field of the Invention
The present invention relates to an electronic driving-torque control system for an automotive vehicle with a continuously variable automatic transmission, often abbreviated to "CVT", in which a required driving torque to be applied to axle driveshafts or drive wheels is determined as a combination of the engine power output and the transmission ratio of the CVT, and specifically to a system capable of providing the enhanced accuracy of driving-torque control in a low-speed range as well as in mid- and high-speed ranges.
2. Description of the Prior Art
In continuously variable automatic transmissions such as a belt type continuously variable automatic transmission, a toroidal type continuously variable automatic transmission, or the like, a desired transmission ratio of the continuously variable automatic transmission (CVT) is first derived or determined depending on both vehicle speed and engine load, for automatic shifting control. Feedback control is then executed so that an actual transmission ratio is adjusted to the desired transmission ratio. During acceleration where the required engine load increases with the driver's accelerator pedal depression, an automatic downshifting operation is executed, so that the actual transmission ratio is adjusted toward near the lowest speed-change ratio with the desired transmission ratio automatically increasing. During driving at low engine loads with the accelerator pedal released, an automatic upshifting operation is executed, so that the actual transmission ratio is adjusted toward near the highest speed-change ratio with the desired transmission ratio automatically decreasing. Japanese Patent Provisional Publication No. 7-172217 discloses techniques for deriving a required driving torque value (a positive wheel torque value) based on both the vehicle speed and the accelerator-pedal depression amount. In recent years, the driving-torque control is often combined with the automatic shifting control as previously discussed, in order to enhance the accuracy of driving-torque control, accounting for better fuel economy. The integrated driving-torque control, in which the electronic engine power output control is integrated into the automatic shifting control, has been disclosed in Japanese Patent Provisional Publication No. 62-110536.